What is the purpose of a test environment for rendering and where can I get one?
I guess you're referring to HDRIs.

There are things to know about how to use them. Among all the tutorials out there, you should watch the Image-based lighting course by Gleb Alexandrov. It's the most up to date and best one.

Now here's where you can find some good ones for free :


Free bundle on HDRI Haven plus all those marked with the green CCO in the corner
in the demo version of Pro-lighting : Skies
Freebies on HDR Maps


Less high-res (usable for lighting and reflections but not for direct background viewing) :


free set on HDR Sets
a bunch on CG Advertising
all skies on HDRI Skies in 2K resolution


I guess you're pretty much covered. Have fun Blending.